op-wheel: add --allow-gaps flag to engine block building commands

op-wheel/commands.go

@@ -78,11 +78,17 @@ var (
 		EnvVar: opservice.PrefixEnvVar(envVarPrefix, "BUILDING_TIME"),
 		Value:  time.Second * 6,
 	}
+	AllowGaps = cli.BoolFlag{
+		Name:   "allow-gaps",
+		Usage:  "allow gaps in block building, like missed slots on the beacon chain.",
+		EnvVar: opservice.PrefixEnvVar(envVarPrefix, "ALLOW_GAPS"),
+	}
 )
 
 func ParseBuildingArgs(ctx *cli.Context) *engine.BlockBuildingSettings {
 	return &engine.BlockBuildingSettings{
 		BlockTime:    ctx.Uint64(BlockTimeFlag.Name),
+		AllowGaps:    ctx.Bool(AllowGaps.Name),
 		Random:       hashFlagValue(RandaoFlag.Name, ctx),
 		FeeRecipient: addrFlagValue(FeeRecipientFlag.Name, ctx),
 		BuildTime:    ctx.Duration(BuildingTime.Name),
@@ -336,7 +342,7 @@ var (
 		Usage: "build the next block using the Engine API",
 		Flags: []cli.Flag{
 			EngineEndpoint, EngineJWTPath,
-			FeeRecipientFlag, RandaoFlag, BlockTimeFlag, BuildingTime,
+			FeeRecipientFlag, RandaoFlag, BlockTimeFlag, BuildingTime, AllowGaps,
 		},
 		// TODO: maybe support transaction and tx pool engine flags, since we use op-geth?
 		// TODO: reorg flag
@@ -362,7 +368,7 @@ var (
 		Description: "The block time can be changed. The execution engine must be synced to a post-Merge state first.",
 		Flags: append(append([]cli.Flag{
 			EngineEndpoint, EngineJWTPath,
-			FeeRecipientFlag, RandaoFlag, BlockTimeFlag, BuildingTime,
+			FeeRecipientFlag, RandaoFlag, BlockTimeFlag, BuildingTime, AllowGaps,
 		}, oplog.CLIFlags(envVarPrefix)...), opmetrics.CLIFlags(envVarPrefix)...),
 		Action: EngineAction(func(ctx *cli.Context, client client.RPC) error {
 			logCfg := oplog.ReadLocalCLIConfig(ctx)

op-wheel/engine/engine.go

@@ -95,21 +95,30 @@ func updateForkchoice(ctx context.Context, client client.RPC, head, safe, finali
 }
 
 type BlockBuildingSettings struct {
-	BlockTime    uint64
+	BlockTime uint64
+	// skip a block; timestamps will still increase in multiples of BlockTime like L1, but there may be gaps.
+	AllowGaps    bool
 	Random       common.Hash
 	FeeRecipient common.Address
 	BuildTime    time.Duration
 }
 
 func BuildBlock(ctx context.Context, client client.RPC, status *StatusData, settings *BlockBuildingSettings) (*engine.ExecutableData, error) {
+	timestamp := status.Head.Time + settings.BlockTime
+	if settings.AllowGaps {
+		now := uint64(time.Now().Unix())
+		if now > timestamp {
+			timestamp = now - ((now - timestamp) % settings.BlockTime)
+		}
+	}
 	var pre engine.ForkChoiceResponse
 	if err := client.CallContext(ctx, &pre, "engine_forkchoiceUpdatedV1",
 		engine.ForkchoiceStateV1{
 			HeadBlockHash:      status.Head.Hash,
 			SafeBlockHash:      status.Safe.Hash,
 			FinalizedBlockHash: status.Finalized.Hash,
 		}, engine.PayloadAttributes{
-			Timestamp:             status.Head.Time + settings.BlockTime,
+			Timestamp:             timestamp,
 			Random:                settings.Random,
 			SuggestedFeeRecipient: settings.FeeRecipient,
 			// TODO: maybe use the L2 fields to hack in tx embedding CLI option?
@@ -210,6 +219,7 @@ func Auto(ctx context.Context, metrics Metricer, client client.RPC, log log.Logg
 
 				payload, err := BuildBlock(ctx, client, status, &BlockBuildingSettings{
 					BlockTime:    settings.BlockTime,
+					AllowGaps:    settings.AllowGaps,
 					Random:       settings.Random,
 					FeeRecipient: settings.FeeRecipient,
 					BuildTime:    buildTime,